Presently, leather is processed in a tannery, where raw hides are converted into leather in a series of chemical and physical operations. These processing operations can be divided into wet operations and dry operations. The wet operations include soaking, liming, deliming, bating and pickling. This is followed by tanning, re-tanning, fatliquoring and dyeing. Following the wet operations, the dry operations are carried out. These comprise finishing the leather, as by coating with film-forming materials.
Leather is conventionally dyed in drums by a so-called exhaust system. Hides or skins are loaded into the drum. Water is fed into the drum and the drum rotated, to cause the hides and water to tumble around inside the drum. Dyestuff is then added. The dyestuff is exhausted, as it is absorbed by the leather. For commercial scale production, the drums are quite large, and may hold 500-1,000 pounds of leather. Consequently, it can take a long time, for example 1-2 hours, for the dyestuff to be exhausted, the time being generally independent of the load.
This conventional technique has a number of disadvantages or problems. Firstly, it is common for the colour to be incorrect. This is only apparent once a complete batch has been processed and dried. Correcting colour errors is difficult. The process is only suitable for processing large batches, and cannot economically be applied to small batches.
Because of these difficulties, more recent proposals have relied upon techniques such as spraying, curtain coating, dip dyeing and roller coating. These techniques are used to dye crust leather.
Spraying is mostly used for colour correcting previously dyed leather. This is because it is difficult to apply large quantities of dye by spraying. If one attempts to apply large quantities, this generates a large quantity of mist and frequently causes the reverse side of the leather to be wetted by the dye as well. In many cases, this is undesirable. Also, the use of a heavy spray causes the conveyors on which the leather is placed to be contaminated, and results in considerable wastage of dye. This, and necessary ventiliation for most removal, can cause pollution problems. Curtain coating is a technique in which the dye is caused to fall as a continuous vertical film or curtain, that is applied to the leather. The intention is that the curtain of dye will apply a uniform curtain to the leather surface. However, in order to form a continuous curtain, it is necessary to add thickening agents to the dye solution. Such thickening agents have the disadvantage of reducing the dye penetration into the leather. In this respect, it is to be noted that relatively small increases in the viscosity of the dye solution can significantly affect the penetration into the leather.
One dip dyeing technique is known as the Multimac method, and the Multimac Dyeing Machine is built by Trockentechnik GmbH of West Germany. In this technique, individual pieces of leather are guided through a bath of dye, and then withdrawn through a pair of samming rolls. This technique has the disadvantage that the leather is dyed on both sides, and it is not possible to simply dye one side. Further, the leather absorbs 100% of moisture and has to be dried and reprocessed, similarly to leather from the drum of the exhaust system dyeing technique.
As regards to roller coating techniques, there are three principal roller arrangements. In the direct method, a gravure roll applies a coating directly to the leather. In a reverse process, a gravure roll again applies a coating directly to the leather, but the direction of rotation of the roll is reversed relative to the direction of movement of the leather. There is also an indirect method, in which the finish is first transferred from a gravure roll to a rubber appicator roll, which applies the finish to the leather.
These roller coating techniques are primarily intended for applying a coating to the surface of dyed leather, which coating can have a high viscosity, rather than applying a dye that is absorbed below the surface of the leather. In order for such roller coaters to work properly, the liquid used has to have a certain viscosity, if it is to be properly retained in the screen of the gravure roll. However, for proper absorption of dye, it needs to have a low viscosity, so that it is readily absorbed to a considerable depth in the leather. Deep absorption of the dye is necessary for good buff and scratch resistance. Further, because of the nature of the application rollers and the viscosities that have to be used, usually only relatively small quantities of material can be applied. Suggested techniques can apply significant quantities of a relatively high viscosity dye but this requires a reverse process. In this context, it should be kept in mind that relatively small differences in viscosity can have a significant effect on absorption.
In general, existing roller techniques cannot apply the required quantities of a low viscosity dye necessary to get deep penetration of the dye.